Hydroelectric power had been considered to be environmentally clean until it was realized that many freshwater reservoirs created through construction of dams across rivers accumulated, at least on a seasonal basis, large amounts of methane. This is because of stagnation and consequent loss of oxygen in the deeper part of the water column that is physically separated from the well-oxygenated surface layer. As methane is approximately 25 times more powerful as a greenhouse gas than carbon dioxide, its emission to the atmosphere from dam-reservoirs can contribute to global warming, argues the fossil-fuel lobby. In fact, based on a study by Ivan Lima and his colleagues from Brazil's National Institute for Space Research, published in a peer-reviewed journal, annual methane emissions from dam-reservoirs in India alone were estimated to be around 33.5 million tonnes. This amounts to about 19% of the country’s global warming emissions from all sources and 28% of methane emission from all large dams of the world, figures that raised a great deal of alarm in the media and among NGOs. However, this “study” was based on imaginary numbers as there were no real data then available on methane concentrations from any Indian freshwater reservoir.

Scientists of the CSIR-National Institute of Oceanography (CSIR-NIO), Goa, led by Gayatree Narvenkar have now made fairly extensive measurements of methane in eight dams-reservoirs spreading across India from the Western Ghats (Selaulim, Supa, Tillari, Koyna and Markandeya) to the central Indo-Gangetic Plain (Rihand) and the foothills of the Himalayas (Bhakra Nangal). Although most of these reservoirs were found to turn anaerobic during the summer, measured dissolved methane concentrations are substantially lower than assumed in the above-mentioned "study". Moreover, accumulation of methane generally occurred below the depths of water intake for power generation and irrigation, and methane concentrations were invariably very low in the well-oxygenated surface layer indicating insignificant emission flux. The study concluded that most of the methane produced in Indian reservoirs is converted to carbon dioxide by methane oxidizing micro-organisms (methanotrophs). These results, published in the journal Environmental Monitoring and Assessment, demonstrate that methane emission to the atmosphere from the Indian dam-reservoirs had been greatly overestimated and that the hydroelectric power in the country appears to be quite green.